1. Field of the Invention
The present invention relates to a vehicle-surroundings monitoring apparatus which is capable of monitoring conditions around a vehicle by means of a plurality of electromagnetic wave transmitter and receiver sections (radar devices) provided on and along the periphery of the vehicle, to thereby inform the vehicle operator of the conditions around the vehicle and automatically restrict or control the operator's driving operation so as to safely avoid any possible obstacles or objects the vehicle may encounter,
2. Description of the Related Art
Japanese Patent Laid-open Publication No. HEI 6-94829 proposes a vehicle cruise controlling apparatus which employs an FM-CW (frequency-modulated continuous wave) radar device to determine a distance to a preceding vehicle so that the vehicle mounting this apparatus (hereinafter "user's vehicle") can operate on an automatic cruising mode while automatically adjusting its velocity in response to a varying vehicle distance to the preceding vehicle or automatically activate the brake when the distance goes below a predetermined limit.
The FM-CW radar device is however disadvantageous in that Doppler effect would prevent the device from accurately determining a distance to a preceding vehicle or object varying in its relative velocity to the user's vehicle. To address this inconvenience, the above-mentioned 6-94829 publication proposes a technique of estimating an accurate distance to a preceding object by transmitting a radio wave signal at a rate of r=f/t (where f represents the predetermined fundamental frequency of the signal, and t represents an estimated time). According to the proposed technique, the transmission rate r is set such that the frequency components of a differential signal resultant from a relative velocity between the object and the radar device corresponds to a distance over which the object should move relative to the radar device at the relative velocity within the estimated time t.
In addition, Japanese Patent Laid-open Publication No. SHO 62-88978 proposes a radar device which transmits a signal obtained by amplitude-modulating a carrier wave signal, determines a relative velocity between a possible object and the radar device on the basis of a frequency difference between the transmitted signal and a received signal reflected from the object, and also determines a distance to the object on the basis of a phase difference between the signal used to amplitude-modulate the carrier wave signal at the transmission end of the radar device and a demodulated signal obtained by demodulating the received signal at the reception end.
In recent years, there has been an increasing demand for an improved vehicle-mounted radar which permits monitoring conditions over an entire range around a vehicle, not just in front thereof, and which also effectively assists the vehicle operator's recognition and judgement in changing a route such as left- or right-turning, changing a lane, putting the vehicle into a garage, etc.